EDIT 4, 5 and 6
8 hours in, any more ideas are welcome :) Maybe this bug is already known and solved, but I get the behaviour I described in edit 2&
I had this issue the other day and solved it by editing the production.rb file
config.assets.compress = true
config.assets.compile = true
config.assets.digest = true
config.assets.initialize_on_precompile = false
Are the asset settings I ended up with. I precompile my assets, while deploying with capistrano.
Before I edited the asset settings, I had exactly the same issue. The .css was being requested and was compiled into my public/assets folder but the server couldnt find the files.